diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index e03f3e7f12c2..5921869949ec 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -454,7 +454,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             freshener: infcx.freshener(),
             intercrate: None,
             inferred_obligations: SnapshotVec::new(),
-            intercrate_ambiguity_causes: Vec::new(),
+            intercrate_ambiguity_causes: None,
             allow_negative_impls
         }
     }